A heuristic function h(n) satisfies the monotone restriction
if for all nodes ni and nj,

h(ni) &le h(nj) + c(ni,nj)

If h(n) satisfies this restriction (similar to the triangle
inequality), then a node never has to be moved from closed
back to open. In this case, closed
could be eliminated, saving storage.

The effectiveness of a heuristic can be expressed as the
effective branching factor, b*.
b* is the inferred branching factor that would produce the actual
number of nodes searched at the solution depth d.